Centrifugal multistage compressors are conventionally known. For example, Patent Document 1 describes a three-stage compressor. The compressor includes a motor, an input shaft having an input gear, a first pinion shaft having a first-second stage pinion gear, a second pinion shaft having a three stage pinion gear, a first stage impeller, a second stage impeller, a third stage impeller, and a step-up gear housing. The number of teeth (pitch diameter) of the input gear is set to be larger than the number of teeth (pitch diameter) of each pinion gear. The motor is connected to the input shaft such that the input shaft is drivable and rotatable. In other words, the input gear and the pinion gears each serve as a step-up gear. The first stage impeller is connected to one end of the first pinion shaft. The second stage impeller is connected to the other end of the first pinion shaft. The third stage impeller is connected to one end of the second pinion shaft. The step-up gear housing places the input gear, the first-second stage pinion gear, and the third stage pinion gear, and has a shape that allows the opposite ends of the input shaft, the opposite ends of the first pinion shaft, and the opposite ends of the second pinion shaft to be exposed. The step-up gear housing supports the input shaft, the first pinion shaft, and the second pinion shaft such that each of the first-second stage pinion gear and the third stage pinion gear is engaged with the input gear. Specifically, the step-up gear housing includes a lower step-up gear housing and an upper step-up gear housing that are separable from and connectable to each other in an up and down direction. The shafts are arranged side-by-side along a division surface between the lower and upper step-up gear housings (on an upper end surface of the lower step-up gear housing). Accordingly, supporting portions for supporting the shafts are formed on the upper end surface of the lower step-up gear housing, thereby facilitating the installation of the shafts (in particular, the input shaft that has the input gear having the large pitch diameter) in the housing. Specifically, the shafts are first placed on the supporting portions formed on the upper end surface of the lower step-up gear housing. Then, the upper step-up gear housing is connected to the lower step-up gear housing from above. Accordingly, the installation of the shafts in the housing is completed.
In the compressor described above, when the motor is driven, the first pinion shaft is rotated through the input shaft, the input gear, and the first-second stage pinion gear, and at the same time, the second pinion shaft is rotated through the input shaft, the input gear, and the third pinion gear. Accordingly, the air compressed by the first stage impeller is further compressed by the second stage impeller, and the air compressed by the second stage impeller is subsequently further compressed by the third stage impeller.
The development of multistage compressors has been recently progressed. However, in the compressor described in Patent Document 1, it is difficult to increase the number of compression stages while preventing complicated installation of the shafts in the housing, a significant increase in size of the housing, and occurrence of poor engagement between the gears.
For example, in the compressor described in Patent Document 1, if the number of compression stages is increased while preventing the complicated installation of the shafts in the housing, an additional pinion shaft will be disposed on the upper end surface of the lower step-up gear housing. In this case, the size of the housing in a direction orthogonal to the shafts disposed on the upper end surface is increased. This becomes prominent as the number of additional pinion shafts (the number of additional compression stages) increases.
In addition, if the number of compression stages is increased while preventing the increase in size of the housing in the orthogonal direction, and the complicated installation of the shafts in the housing, for example, the upper step-up gear housing will be further divided in the up and down direction, and an additional pinion shaft will be placed along a division surface of the upper step-up gear housing. In this case, the number of divisions of the housing is increased. Therefore, tolerance generated in assembling the housing is accumulated, whereby the poor engagement between the gears may occur.